It is normal to connect together in the same housing several motor vehicle lighting or signalling functions, so as to simplify the electrical wiring for these various functions.
Each function comprises a light source, a reflector and possibly a glass, these various elements being arranged so as to provide a lighting or signalling beam whose geometric and photometric characteristics must be in accordance with various regulations. Each function thus requires a minimum volume for its installation in a particular lighting or signalling device.
However, the volume available for installing lighting or signalling devices is becoming smaller and smaller, both at the front and at the rear of the motor vehicle. This is because the constraints of aerodynamics and the designs of the stylists result in shapes which are very different from those which result solely from technical considerations.
Thus, in many cases, the space available for grouping together functions in the same housing is insufficient, and it is then necessary to distribute the various functions in different housings, and hence a rise in the cost and an increase in the wiring and assembly time.
In order to resolve this problem, it is known how to use light guides. There is known for example from the document EP-A-1 243 467 a headlight comprising an elliptical module. A circular light guide surrounds the lens of the elliptical module and comprises a surface for decoupling the light cooperating with a cylindrical reflector and an annular reflector for directing the rays issuing from the light guide forwards. The purpose of such an arrangement is to increase the apparent surface of the elliptical module.
There is also known, from the document EP-A-1 036 979, a light guide provided with a line of micro-etchings on its rear face, a reflector being disposed at the rear of this light guide, the line of micro-etchings being placed substantially at the focus of the reflector. A light guide of this type is intended to be associated with other identical light guides disposed side by side in order to form a light-emitting surface.
Another light guide produced according to the prior art is depicted in FIGS. 1 to 3.
The light guide 10 comprises here two ends 12 and 14. The light guide 10 is in particular intended to channel light rays which enter through its ends 12 or 14.
The light guide 10 is obtained by the movement of a roughly circular transverse elementary section 16 along a director line 18. The director line 18 is here situated substantially at the centre of the elementary section 16 and the elementary section 16 is perpendicular to the director line 18.
The form of the director line 18 is defined freely by a person skilled in the art in particular according to aesthetic constraints. As depicted in FIG. 1, the director line 18 here is in the form of a loop.
In the description which follows, a longitudinal, vertical and transverse orientation will be adopted non-limitingly, which is indicated by the trihedron L, V, T in FIGS. 2 and 5, and which will be applied locally to all the elementary sections 16 of the light guide 10. Thus the longitudinal direction L is always perpendicular to each of the elementary sections 16, whatever the overall form of the light guide 10. Thus the vertical direction V as defined is able to vary with respect to a reference frame related for example to the motor vehicle according to the form of the director line 18.
By convention, the transverse direction T is oriented from the rear, on the right in FIGS. 2 and 5 to 9, towards the front, on the left in FIGS. 2 and 5 to 9. In the examples depicted in FIGS. 1, 3 and 4, the transverse direction T is fixed with respect to a reference frame related to the motor vehicle, and is indicated by the arrow T in FIGS. 1, 3 and 4.
As illustrated in FIG. 2, the contour 20 of the elementary section 16 comprises a front peripheral lighting portion 22 and a rear peripheral portion 24.
The rear peripheral portion 24 comprises a rear segment 26 for diffusing the light. The length of the rear segment 26 being small compared with the diameter of the elementary section 16, the rear segment 26 will be considered to be a point in the remainder of the description. A principal optical axis 28 extends transversely from the rear diffusion segment 26 as far as the front lighting portion 22.
As depicted in FIG. 3, all the rear diffusion segments 26 create a rear light diffusion line which extends parallel to the director line 18 over at least a length of the light guide 10.
The light rays enter the light guide 10 through at least one of its ends 12 or 14. The light rays are here emitted by a light source, for example a light-emitting diode 30, also referred to as an “LED”, which is situated opposite the end 14.
In order to channel the light, the light guide 10 is formed from a transparent material such as glass or plastic, which has a higher refractive index than the medium in which the light guide 10 is immersed, in particular air.
As depicted in FIG. 3, the light rays Rc entering through the end 14 of the light guide 10 are channelled inside the light guide 10 by successive total reflections on the contour 20 of the elementary sections 16, which are due to the difference in refractive indices between the air and the light guide 10, in the general direction of the director line 18.
When these channelled rays Rc reach a rear diffusion segment 26, they are diffused in all directions and in particular:    towards the rear, the rays thus diffused Rd then emerge outside the light guide 10;    and towards the front inside the light guide 10.
The rays Re diffused towards the front which are included in a given solid angle around the principal optical axis 28 reach the front lighting portion 22 of the light guide 10 with an angle of incidence such that they are refracted and thus emerge outside the light guide 10.
The front lighting portion 22 thus fulfils the required lighting or signalling function for the motor vehicle.
A light guide 10 of this type makes it possible to fulfil a lighting or signalling function meeting the constraints of style and size of the vehicle.
However, a not insignificant part of the light is diffused towards the rear of the light guide 10, as indicated by the rays Rd in FIG. 3. Thus the light projected towards the front by the light guide 10 does not have optimum intensity compared with the light intensity entering through the end face 14 of the light guide 10.